1. Technical Field
The invention concerns a signal amplifier circuit with symmetrical inputs and outputs whereby the signal is routed continuously and symmetrically from the inputs to the outputs. Such amplifiers in particular can be used as both line drivers and line receivers in symmetrical signal lines.
2. Discussion of Related Art
Telecommunications and sound studio technologies mostly use symmetrical signal paths to transmit signals through lines in order to prevent external interference signals from the environment to gain access into the useful signal. It is known that external electric and or magnetic fields in lengthy signal paths in the environment of an electronic installation, for example from its own power supply, can gain access into the useful signal as parts of superimposed external signals. Interfering signal components can also gain access into the signal paths through the power connections themselves and through thermal effects. The symmetrical routing of signals through lengthy lines between parts of the installation is always advantageous. Among other things this is also important for laying signal lines in an automobile, for example to make an intercom connection for the vehicle's driver in the front of the car with a radio telephone located in the rear of the car. As a rule such a line extends for several meters and must not be sensitive to extreme interferences from the remaining on-board electronics, such as the ignition for example.
Such connections are usually made with twisted lines. In that case only the difference in the signals between the two signal paths is processed and reproduced. All external signals which enter both signal paths capacitively or inductively by common-mode or via the power supply, are suppressed through subtraction. A measure of the quality of the symmetrical signal routing is the so-called common-mode rejection (in-phase suppression). It depends strongly on the tolerances of the respective pairs of components in both signal paths.
A so-called "electrometer subtracter" or also an "instrument amplifier" for measuring potential differences are known from the technical literature, such as for example the reference book: Semiconductor Circuit Technology/U. Tietze; Ch. Schenk--5, revised edition--Berlin, Heidelberg, New York: Springer, 1980, section 25.1.2 and illustration 25.3, and from EP 0,453,680 as well. As shown in FIG. 1, it essentially contains the three operational amplifiers OA 1 to OA 3. The operational amplifier OA 3 and the pairs of resistors R 14/R24 and R15/R25 form a subtracter with an asymmetrical signal output O.sub.ASYM. The operational amplifiers OA 1 and OA 2 respectively form non-inverting amplifiers and are located before the inputs of the subtracter. Each non-inverting amplifier has a feedback resistor R.sub.F 1 or R.sub.F 2 between its output and the inverting input. In the interest of a high common-mode rejection, the resistors R.sub.F 1 and R.sub.F 2 as well as the pairs of resistors R14/R24 and R15/R25 must be matched as precisely as possible.
The inverting inputs of both amplifiers are interconnected by a joint resistors R 3 which is used to adjust the amplification.
It is an advantage that the resistor R 3 in both amplifiers OA 1, OA 2 can be used to adjust the joint amplification factor. Thus this resistor alone can be used to adjust the differential gain of the instrument amplifier. The additional effect of the amplifiers OA 1, OA 2 is that the instrument amplifier has two identical signal inputs, and that the parameters of the subtracter, particularly the common-mode rejection, are independent of the internal resistance of the signal source. The common-mode amplification of the non-inverting amplifiers OA 1 and OA 2 has the value of 1, regardless of the adjusted differential amplification.
It is a disadvantage that the instrument amplifier has only one asymmetrical output. This requires at least one further operational amplifier in order to also provide an inverted signal in addition to the non-inverted signal.
Another disadvantage of the known solution is that a high common-mode rejection can only be achieved when the corresponding circuit components in every branch of the circuit, including the inductive and capacitive line components of the external and internal circuit, have close tolerances. In practice this condition can only be attained with great effort since in addition to paired components with close tolerances, the signal paths also require the line routing to be symmetrical. Particularly if for example an IC (Integrated Circuit) with four integrated operational amplifiers is used in the circuit, the symmetry of the external circuit can only be approximately achieved even if it has been carefully constructed, because the signal branches are already different due to the number of active components, where different phase responses can hardly be avoided. The full circuit furthermore occupies a relatively large area of the circuit board.
It is also known to construct an instrument amplifier which includes all passive components in the form of an IC. The Maxim Integrated Products Company of Sunnyvale, Calif. already produces a pair of IC's for transmitting video or telecommunication signals via pairs of broad-band leads, which comprise a line driver IC MAX 4147 and a line receiver IC MAX 4144 or MAX 4146. The line receiver IC is, also designed as an instrument amplifier according to FIG. 1 with symmetrical differential inputs and an asymmetrical output, and contains pairs of laser-trimmed thin film resistors. In contrast thereto the line driver IC only contains two operational amplifiers connected as non-inverting amplifiers according to the two amplifiers OA 1, OA 2, the feedback resistors according to the resistors R.sub.F 1 and R.sub.F 2 and the joint resistor according to resistor R 3 in FIG. 1, but no subtracter. Therefore the signal being transmitted through the line still contains common-mode parts. These are only eliminated in the line receiver by means of the signal subtraction. In this way the common-mode parts of the signal from the line driver IC reach the line receiver IC in an unimpeded manner via the pair of lines, and are then amplified once more. In the presence of large common-mode amplitudes this can lead to over-modulation of the line receiver IC. This can be avoided by installing an additional inductive transmitter between the line driver IC and the line.